FIGS. 1a, 1b and 1c are diagrammatic views illustrating application of load to a conventional rail pad and FIGS. 2a, 2b and 2c are similar diagrammatic views of a recessed pad of harder material.
To further explain the problem overcome by this invention, reference is made to FIGS. 1A, B and C which illustrate a conventional thick rail pad which is used to provide high force attenuation. Where the pad is located on curved track and the load is applied along the line D as shown in 1A the pad initially will deflect vertically as shown in 1B which results in a reduction in the bearing width E--E being reduced. This reduction in bearing width means that greater deflection of the pad results which in turn results in a further reduction of bearing width on the field edge side as shown in F--F in FIG. 1C. This leads to instability of the rail seat and excessive wear on the rail pad.
In FIGS. 2A to 2C, a similar situation is shown except a harder material is used for the pad but a recess in the pad under the rail provides higher attenuation. The bearing width G--G on the field side of the pad under a diagonal load D remains virtually constant.
In order to optimize the resistance to damage by the rail and the attenuation of the load this invention provides that the material of the pad is resilient, flexible and has an optimum combination of hardness, ductility or resilience, resistance to cutting and a high spring rate. Preferably there should be used a natural or synthetic rubber blended to give a hardness of 68 to 90 DURO-A with abrasion and cutting resistance and resistance to permanent compression under high load durations (creep resistance).
A rail pad of such material would of itself have insufficient force attenuation. However, in accordance with this invention a pad of material having hardness within the range of 68 to 85 DURO-A and abrasion and cutting resistance is used wherein the portions of the pad underlying the rail are provided with grooves or recesses to provide higher force attenuation than the edge portions of the pad.
In the edge portions of the pad it is preferred that no grooves or recesses be provided which would reduce the bearing capacity of the pad. This means that the edge portions will have a high spring rate (the same as the selected material). However, the central portion of the pad must have high attenuation and it is essential to this invention that the central portion of the pad have a low load bearing capacity and a correspondingly low spring rate.
This may be achieved by providing a completely hollow central region with no material. If material is provided there is no need for it to contact either the rail or the rail tie and can take the form of a connecting web.
If surface contact between the pad and the rail or rail tie is required the area of contact should be low and the structure of the pad be such that the spring rate is low and high attenuation will occur. This can be provided by having the central region corrugated with grooves having sloping sides to reduce the bearing capacity of this portion of the pad, because the sloping walls of the corrugations when under load will be in shear, not compression. It is possible using this construction to use corrugations parallel to the longitudinal dimension of the rail extending completely across the pad. It is also possible to provide a lower spring rate in the centre of the pad by providing wide grooves of a depth greater than 50% of the pad thickness.
When thicker high force attenuating rail pads are used, considerable vertical deflection of the pad will occur. This deflection will of course vary with the hard ness of the pad material but is also related to pad thickness. The thicker the pad the greater the vertical deflection. This vertical deflection under load causes the pad to deform in a lateral direction from under the rail seat with each load pulse arising from the passage of a train wheel. However, the portion of the pad located between the rail and the rail clamp support shoulder is completely enclosed by the clamp support and the rail and this restricts such a lateral deformation. Instead the pad tends to deform upwardly under the clamp insulator which lies partly on the rail flange and partly on the rail pad between the flange and the clamp support. This upward force and movement on the clamp insulator leads in some cases to an early fatigue failure of the clamp insulator.
To overcome this particular difficulty this invention provides that a recess be incorporated in the rail pad in the portion of the pad which abuts the clamp support. This recess can be a hollow in the upper or lower surface of the pad or grooves into the upper or lower surface which have the effect of reducing the volume of the pad in that portion of the pad.
Such a recess will provide sufficient room for the pad to deform into, under load, without applying force to the clamp insulator. Preferably a recess is provided in the upper surface which lies beneath and within the boundary of the clamp insulator. It is preferred that the recess represents at least 10% preferably 25% of the volume of the pad lying between the edge of the clamp support which is parallel to the rail and the rail edge. The volume of the recess may be greater than 50% of the volume of this portion of the pad but no further advantage is obtained.